The present invention relates to an electric discharge machine for making holes of various shapes in a workpiece by advancing a tool electrode towards the workpiece while causing an electric discharge between the tool electrode and the workpiece.
Electric discharge machines are widely used to accurately machine solid conductive workpieces into molds of various shapes. The workpiece is normally placed in a work. tank and fixed to a table movable in the horizontal plane. The tool electrode is attached to a lower end of a quill movable in the vertical direction using a suitable electrode holder. The tool electrode is manufactured from a material that is easy to cut, such as, for example, copper or graphite. In preparation for machining the workpiece, the work tank is filled with dielectric fluid such as kerosene, and the tool electrode is positioned extremely close to the workpiece. A space between the tool electrode and the workpiece is called a gap, and size of this gap is controlled to be between a few xcexcm to a few tens of xcexcm. If a power pulse is applied between the tool electrode and the workpiece during the ON-time, the insulating characteristics of the dielectric fluid in the gap break down and an electric discharge occurs. The material of the workpiece evaporates or melts as a result of the heat from the electric discharge and becomes entrained in the dielectric fluid. Upon completion of the ON-time, application of the power pulse is suspended during the OFF-time and the insulating properties of dielectric fluid in the gap are restored. Electric discharge machines ordinarily repeatedly apply the power pulse to the gap with the ON-time and the OFF-time controlled between 1 xcexcsec to several tens of msec. As the gap is maintained at a constant size, the tool electrode is gradually moved downwards towards the workpiece in accordance with removal of the workpiece material. In a coordinate system for positioning the tool electrode relative to the workpiece, a line representing an amount of linear movement of the tool electrode towards the workpiece is called the Z axis from a control viewpoint. In many electric discharge machines, the Z axis number normally represents a position of the tool electrode in the vertical direction. Since the electric discharge machine removes a microscopic amount of material from the workpiece at a time without the tool electrode actually coming into contact with the workpiece, a cavity having a desired surface roughness is formed in the workpiece with good accuracy. The cavity is complementary in size and shape to the tool electrode, which means that various tool electrodes are used according to the shape of the cavity required. In order to make a large cavity, a large tool electrode becomes necessary, and electric discharge machines capable of holding a tool electrode in excess of 100 Kg on the quill are known.
A flushing operation for producing a flow of dielectric fluid through the gap is necessary in order to rinse fragments that have been removed from the workpiece from the gap. The flushing operation prevents undesirable secondary discharge between the tool electrode and fragments that have been removed from the workpiece, and contributes to restoration of the insulating properties of the dielectric fluid during the OFF-time. In preparation for machining of the workpiece, a skilled operator will make holes for introducing fresh dielectric fluid into the gap and suctioning used dielectric fluid from the gap in the tool electrode and the workpiece. Flushing is the key to faster and better precision electric discharge machining, but skill and experience are required in order to produce uniform flow across the entire gap according to the shape of a required cavity. Depending on the situation, it may not be desirable to form flushing holes in the workpiece, or there might be restrictions in forming those types of holes in the tool electrode. For example, in the case where an operator is making a deep cavity having an elongated opening in a workpiece, a thin rib-shaped tool electrode is used. Because it is difficult to form flushing holes in such a tool electrode, an injection system is normally used to inject dielectric fluid from the side of the tool electrode towards the gap. However, an injection system can not sufficiently remove contaminated dielectric fluid from the gap as the cavity being formed in the workpiece becomes deeper. An operation known as a xe2x80x9cjumpxe2x80x9d is known for compensating for this insufficient flushing operation. The jump operation involves periodically raising and then lowering the tool electrode rapidly in the Z axis direction, and drives out almost all of the contaminated dielectric fluid from the cavity in the workpiece. Conventionally, the tool electrode moves at a speed of several hundred mm/min during the jump operation. If the reciprocating distance of the tool electrode is large, more fresh dielectric fluid flows in to the gap, and more contaminated fluid is removed from the gap. The tool electrode is preferably raised up by at least the depth of the cavity being machined in the workpiece. However, since no material is removed from the workpiece during the jump operation, performing the jump operation too frequently will adversely lower the stock removal rate. In order to carry out a jump operation with a large amount of movement that does not cause a lowering of the stock removal rate, the tool electrode is preferably made to move at high speed and with an acceleration and deceleration in excess of 1 G.
An object of the present invention is to provide an electric discharge machining apparatus that can effectively wash fragments removed from a workpiece away from a gap without causing a reduction in stock removal rate, even when machining a deep cavity using a thin tool electrode.
Another object of the present invention is to provide an electric discharge machine that is as compact as possible.
Additional objects of the invention will be set forth in the description that follows, and will become apparent to those skilled in the art upon practicing the invention.
In order to achieve the above and other objects, an electric discharge machining apparatus according to one aspect of the present invention for machining a workpiece by moving a tool electrode along a Z axis towards the workpiece while causing an electric discharge between the workpiece and the tool electrode comprises:
a first movable body movable along the Z axis;
a ball screw;
a motor for causing rotation of the ball screw;
a nut, attached to the first movable body, threadingly engaging the ball screw;
a second movable body movable along the Z axis relative to the first movable body, capable of having the tool electrode attached thereto; and
a linear motor for moving the second movable body.
The linear motor preferably comprises a stator attached to the first movable body, and a mover attached to the second movable body.
The first movable body preferably comprises an electrode holder to which the tool electrode can be attached.
The second movable body is preferably provided coaxially with the first movable body.
In accordance with another aspect of the invention, an electric discharge machining apparatus of the present invention for machining a workpiece by moving a tool electrode along a Z axis towards the workpiece while causing an electric discharge between the workpiece and the tool electrode comprises:
a first movable body movable along the Z axis and having a first electrode holder to which the tool electrode can be attached;
a ball screw;
a motor for causing rotation of the ball screw;
a nut, attached to the first movable body, threadingly engaging the ball screw;
a second movable body movable along the Z axis relative to the first movable body, and having a second electrode holder to which the tool electrode can attached; and
a linear motor for moving the second movable body.
Accordingly, the tool electrode can be selectively attached to one of the first and second electrode holders. A detector is preferably provided for detecting selection of the electrode holder.